A method is being developed for the estimation of local rates of protein synthesis in brain in vivo. The method is based on the use of L-[1-14C]leucine as a tracer for the incorporation of leucine into protein. Six kinetic models for the behavior of leucine on brain have been designed. By mathematical analysis of the kinetics of exchange of the amino acid between plasma and the tissue pool(s) and its incorporation into protein, equations have been derived for each model that define the rate of amino acid incorporation into protein in terms of the time course of plasms-specific activity, final tissue concentration of 14C, and experimentally determined kinetic constants. Tissue concentrations of 14C are determined by quantitative autoradiography. Experiments are being carried out to test the validity of the various models. The method is currently being applied to studies of aging, development, hypothyroidism, regeneration, and sleep.